The present invention concerns an optoelectronic distance measurement device for determining the distance of an object in a measurement area from the device.
Such optoelectronic distance measurement devices are used, for example, for optoelectronically determining the distance from the measurement device of an object that is located in a given measurement area. This allows one, for example, to classify objects on a conveyor band on the basis of their size or to determine the fill level of containers, and to forward this information to a central monitoring location or unit.
Known optoelectronic distance measurement devices capable of making such distance determinations can be roughly divided into two groups based on their underlying technology. One group of such measurement devices determines the distance by triangulation, as is described, for example, in German patent publication DE 35 13 671 C2. The triangulation method directs a focused light beam from the measurement device towards the object. Upon impact, a portion of the light beam is reflected in the form of a scattered light cone. A section of the scattered light cone is sensed by a light receiver that is located at a predefined distance from the light emitter. The light receiver used in the triangulation device can determine the angle at which the light reflected by the object strikes the light receiver.
Light emitter, object and light receiver form a triangle. The distance between the light emitter and the light receiver is known.
The angle between the emitted and received light beams measured by the light receiver can be used to determine the distance to the object.
The second group of such distance measurement devices uses the elapsed light flight time to determine the distance. As is described in DE 40 02 356 C1, for example, a light beam is directed from a light source in the direction of a measurement length. If the light beam strikes an object, a certain portion of the light is reflected back in the direction of the light emitter. A light receiver is arranged in the immediate vicinity of the light source, receives the light reflected by the object, and converts it into corresponding electrical signals. The distance of the object from the distance measurement device can be determined from the elapsed time between the emission of the light beam and the receipt of the reflected light beam by taking the speed of light into consideration. Such an elapsed time distance measurement can employ a phase measurement process, as is described, for example, in DE 40 02 356 C1, or a pulse length time measurement process, as is described, for example, in DE 43 40 756 C2.
The word “light” as used herein is not limited to visible light. “Light” refers generally to electromagnetic radiation, such as UV-light, IR-light, as well as visible light, all of which are commonly used in connection with optoelectronic sensors.
A disadvantage of the triangulation method for determining distance is that the measurement devices have a relatively low accuracy, especially when the base distance between the light emitter and the light receiver is small. Increasing this distance to improve accuracy, however, appreciably increases the size of the measurement device. Distance measurement devices which determine the distance of the object on the basis of elapsed time (pulse running time or phase measurement) are costly to produce because of the very short times that must be measured with a high degree of accuracy. As a result, such distance measurement devices are not used due to their cost.